TOMS435 is a FORTRAN77 library which implements ACM TOMS algorithm 435, which evaluates the modified incomplete Gamma function.
The text of many ACM TOMS algorithms is available online through ACM: http://www.acm.org/pubs/calgo or NETLIB: http://www.netlib.org/toms/index.html.
TOMS435 is available in a FORTRAN77 version.
ASA032, a FORTRAN77 library which evaluates the incomplete Gamma function.
ASA147, a FORTRAN77 library which evaluates the incomplete Gamma function.
ASA239, a FORTRAN77 library which evaluates the incomplete Gamma function.
TEST_VALUES, a FORTRAN77 library which contains routines which return sample values of various functions, including the modified beta function, and the logarithm of the gamma function.
You can go up one level to the FORTRAN77 source codes.